As is known, double-cylinder circular machines for knitting hosiery generally comprise a lower needle cylinder which has a vertical axis and an upper needle cylinder arranged upwardly and coaxially with respect to the lower needle cylinder; said cylinders can be actuated rigidly with each other with a rotary motion about the common axis.
Multiple axial slots are formed on the lateral surface of the lower needle cylinder and on the lateral surface of the upper needle cylinder. The axial slots of the upper needle cylinder are aligned with the axial slots of the lower needle cylinder. Each of the axial slots of the lower needle cylinder generally accommodates, starting from the bottom, a selector and a slider, while each of the axial slots of the upper needle cylinder accommodates a slider. Between the two needle cylinders, i.e., in the knitting region, in each of the axial slots, there is a needle which is provided with two tips or heads, respectively an upper head and a lower head; depending on whether one wishes to provide plain or purl stitches, said needle is moved within the lower needle cylinder so that it knits with its upper tip or in the upper needle cylinder so that it knits with its lower tip.
Since the needle is not provided with a heel, it is actuated by means of the slider arranged in the lower needle cylinder or by means of the slider arranged in the upper needle cylinder, depending on whether it has to form plain or purl stitches.
The sliders currently used in double-cylinder circular knitting machines for hosiery are constituted generally by an elongated laminar body, which has a first longitudinal side designed to rest on the bottom of the axial slot formed on the lateral surface of the lower needle cylinder or on the lateral surface of the upper needle cylinder.
Such sliders are further provided with two heels, which are mutually spaced along the length of the slider and protrude transversely from a second longitudinal side of the slider which lies opposite the first side.
These heels are used to produce the movement of the slider along the corresponding axial slot of the lower or upper needle cylinder so as to produce the actuation of the needle, which is associated with said slider, in the various kinds of knitting of the machine or to transfer the needle from one needle cylinder to the other.
The slider is further provided, on its first longitudinal side, i.e., on its side directed toward the bottom of the axial slot within which it is accommodated, with a hook-shaped tab, which engages the lower head of the needle or the upper head depending on whether the slider is in the lower needle cylinder or in the upper needle cylinder.
Multiple cams for actuating the sliders are arranged around the lateral surface of the lower needle cylinder and around the lateral surface of the upper needle cylinder and form a series of paths, with which the heels of the sliders engage when the needle cylinders are actuated with a rotary motion about their own axis with respect to said cams. The paths formed by the cams are shaped so as to produce the movement of the sliders along the axial slots of the needle cylinders in which they are accommodated and consequently produce the actuation of the needles associated therewith.
In currently commercially available double-cylinder circular knitting machines for hosiery or other knitted articles, many of the cams that form the paths for the heels of the sliders are provided so that they can move along a radial direction with respect to the needle cylinders, so that they can be moved from an active position, in which they are close to the needle cylinders so that they are engaged by the heels of the sliders, to an inactive position, in which they are spaced from the needle cylinders so as to not interfere with the heels of the sliders, or vice versa, in order to allow to vary the paths for the heels of the sliders and consequently vary the kinds of knitting that the machine can perform.
In particular, in these machines, in order to transfer a needle from the upper needle cylinder to the lower needle cylinder or vice versa, slider actuation cams are used which can move on command toward and away from the axis of the needle cylinders.
More particularly, when it is necessary to transfer the needles from one needle cylinder to the other, the needles that must be transferred from the lower needle cylinder to the upper needle cylinder are selected beforehand by selecting the corresponding sliders arranged in the lower needle cylinder by means of an appropriately provided selection device, which faces the lateral surface of the lower needle cylinder, and the actuation cams of the sliders required for the transfer operation are moved into the active position.
Subsequently, both the sliders arranged in the lower needle cylinder and the sliders arranged in the upper needle cylinder engage, with one of their heels, respectively a first lower movable lifting cam and a first upper movable lowering cam, so that the sliders arranged in the lower needle cylinder and the sliders arranged in the upper needle cylinder move mutually closer, reaching a position for engaging the head of the corresponding needle. This mutual approach, as a consequence of the particular shape of the end of the sliders that can engage the head of the corresponding needle, produces the oscillation, on a radial plane of the needle cylinder, of the sliders that were not engaged with the corresponding needle, causing their longitudinal end which can engage the corresponding needle to move away from the bottom of the corresponding axial slot. These sliders subsequently engage fixed pressers, which face laterally the lateral surface of the needle cylinders and cause the oscillation of the sliders in the opposite direction, i.e., moving their end which can engage the needle toward the bottom of the corresponding axial slot, thus achieving simultaneous engagement with the heads of the needle both of the sliders arranged in the lower needle cylinder and of the sliders arranged in the upper needle cylinder. The sliders arranged in the upper needle cylinder thus engage another presser, which faces the lateral surface of the upper needle cylinder and produces the oscillation of the sliders arranged in the upper needle cylinder, moving their end which can engage the upper head of the needle away from the bottom of the corresponding axial slot, disengaging them from said upper head of the needle. The sliders arranged in the lower needle cylinder therefore engage a first lower movable lowering cam, which produces their lowering, while the sliders arranged in the upper needle cylinder engage an upper fixed lifting cam. In this manner, the needles are engaged exclusively with the sliders arranged in the lower needle cylinder. Directly after the first lower movable lowering cam, the sliders that were previously selected, i.e., the sliders that must transfer the corresponding needle from the lower needle cylinder to the upper needle cylinder, are raised by the corresponding selector, while the sliders arranged in the upper needle cylinder are lowered by virtue of the action of a second upper movable lowering cam. This lowering achieves the engagement of the sliders arranged in the upper needle cylinder with the head of the needles that were raised by the lifting of the sliders arranged in the lower needle cylinder, with the consequent oscillation of the sliders arranged in the upper needle cylinder which moves their end that can engage the head of the needle away from the bottom of the corresponding axial slot. These sliders are thus made to oscillate in the opposite direction by engagement with another presser, which faces the lateral surface of the upper needle cylinder, and engage the upper head of the corresponding needle. The sliders arranged in the lower needle cylinder that were raised previously encounter another presser, which faces the lateral surface of the lower needle cylinder and produces their oscillation, with consequent spacing of their end which can engage the lower head of the needle away from the bottom of the corresponding axial slot.
The sliders arranged in the upper needle cylinder that have engaged the upper head of the needle are then raised as a consequence of engagement with another upper fixed lifting cam, which faces the lateral surface of the upper needle cylinder.
At this point, the transfer of preset needles from the lower needle cylinder to the upper needle cylinder is completed, while the other needles, which have not been selected, remain engaged with the sliders in the lower needle cylinder.
The presence of movable cams for actuating the machine, in particular for transferring the needles from one needle cylinder to the other of the machine, is unwanted, since it increases considerably the complexity of the actuation of the machine and forces the adoption of particular control devices in order to allow correct restarting of the machine if electric power accidentally fails.
Moreover, the fact that in known types of machines the needles are transferred from one needle cylinder to the other primarily by transferring all the needles from the upper needle cylinder to the lower needle cylinder requires the use of a rather large needle cylinder sector, causing problems for accommodating other cams or other devices required for correct operation of the machine.